1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image by scanning a photosensitive member with a light beam, such as a laser beam printer.
2. Description of the Related Art
FIG. 4 is a perspective view of an example of a beam scanning apparatus.
Referring to FIG. 4, a laser unit 2 conducts ON/OFF modulation of a laser beam in accordance with an image signal (VDO signal) 1. Motor 4 rotates a rotary polygon mirror 5, which deflects laser beam 3 emitted from the laser unit 2, at a constant speed. Imaging lens 6 focuses a laser beam 7 deflected by the polygon mirror 5 onto a photosensitive drum 8. Beam detector 9 detects whether or not the laser beam 7, which has been deflected by the polygon mirror 5, is disposed on the optical path between a light receiving start position A of the rotating photosensitive drum 8 and the polygon mirror 5, and outputs a beam detection signal (BD signal) 11 in response to the detection of the laser beam 7. A photo-electric conversion element, for example, a photodiode, can be used as the beam detector 9. A latent image is formed on the photosensitive drum 8, the latent image is developed by an unshown developing device, and the developed image is transferred to a transfer sheet 12 to form an image.
The laser unit 2 outputs the laser beam 3 in accordance with the VDO signal 1. The ON/OFF modulated laser beam 3 is deflected by the polygon mirror 5, and focused onto the photosensitive drum 8 through the imaging lens 6. Since the polygon mirror 5 is rotated by the motor 4 at a constant speed, the imaging position of the deflected laser beam 7 is shifted in the main scanning direction AA on the photosensitive drum 8 (horizontal scanning).
A latent image is formed on the photosensitive drum 8 serving as a recording medium is exposed with the laser beam 7. As the photosensitive drum 8 is rotated in the direction of the arrow BB, the latent image formed on the photosensitive drum 8 is developed by the unillustrated developing device (not shown), and then the developed image is transferred onto the transfer sheet 12. The beam detector 9 outputs the BD signal 11 when detecting the laser beam 7 previous to the horizontal scanning with the laser beam 7. Based on the BD signal 11, an unillustrated control unit controls the timing of transferring the VDO signal 1 to the laser unit 2. Thereby, the VDO signal 1 for one scanning line is transferred to the laser unit 2 in synchronization with the BD signal 11, and an image is reproduced on the transfer sheet 12 in accordance with the above procedures. The exposure start position in the main scanning direction AA on the photosensitive drum 8 is determined by the BD signal 11.
FIG. 5 is a timing chart explaining the generation timing of the BD signal 11 and the VDO signal 1.
Since the laser beam 3 is emitted at a timing F in response to a VDO signal 1 which is ON to detect the exposure start position, the beam detector 9 outputs a BD signal 11. After a predetermined time has elapsed after the output of the BD signal 11, the VDO signal 1 is turned off, and image data G, which is to be printed, is output as VDO signal 1 after a period of "a" of a time T1 has elapsed after the output of BD signal 11. Furthermore, a VDO signal 1 is output after a time T1+T2 has passed from the output of the BD signal 11. The above operation is repeated for every line, thereby printing images for one page.
The user can switch the print density as necessity requires in some of laser beam printers having the above construction. The switching of the print density is generally carried out by the following method.
For example, in a laser beam printer, if the print density is increased, the pixel density in the vertical direction of an image is heightened by increasing the number of rotations of the motor 4 for rotating the polygon mirror 5, by increasing the horizontal scanning density of the laser beam 7. Furthermore the pixel density in the horizontal direction is heightened by increasing the clock frequency of the VDO signal 1 for modulating the laser.
In a case that the diameter of the laser beam on the photosensitive drum 8 is constant even if the pixel density is switched, it has a detrimental effect on image quality (for example, a printed character is deformed) at some pixel densities. Therefore, the laser power may be reduced so as to make the diameter of the laser beam on the photosensitive drum 8 equivalently small in printing at high pixel density.
However, when the laser power is decreased, the quantity of light reaching the beam detector 9 is reduced and a sufficient beam position detection signal cannot be obtained. Therefore, the range in which the pixel density can be changed is extremely limited, or when the pixel density is too high, the BD signal 11 serving as an output signal of the beam detector 9 may be missed and thus a proper image cannot be printed.